Selective Barrier to Diffusion

This selectivity can be explained by the different dissociation constant [63], For instance, the dissociation constant of ascorbic acid (pAia = 4.1) is of three orders of magnitude more acidic than the first dissociation constant of quercetin (pAi, = 7.0). At pH 6 or lower, quercetin is, in large extent, undissociated, while ascorbic acid is partially dissociated. In this condition of pH, quercetin is made hydrophobic and its adsorption on nylon is promoted instead, ascorbic acid is made hydrophilic and its adsorption on nylon is prevented. Also, the electrostatic repulsion with the negative charge present on the nylon structure prevents its adsorption on the polymer. As a result, quercetin is adsorbed on the 

The selective protection of the membrane can also be modulated by changing the pH of the solution. For example, the behavior of quercetin at pH higher than 7 is opposite. In this condition of pH, quercetin becomes dissociated and its hydrophobicity decreases. As a result, the quercetin adsorption on the membrane decreases linearly and above pH 10 is not present. 

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Davson, H. (1989). Biological membranes as selective barriers to diffusion of molecules (Tosteson, D.C., Ed.), pp. 15-50. In Membrane Transport People and Ideas. American Physiological Society, Bethesda, MD. 

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